The present invention relates to devices known variously as traversing machines, positioning devices, actuators, etc. More particularly the invention relates to such devices with the ability to traverse along more than a single axis and that use optical encoder systems to determine position and orientation of the stage.
X-Y motors, with a stage supported by some means that permits movement along two perpendicular axes, are known. Displacement information, either incremental, as provided by optical encoders or interferometers, or absolute, as provided by resistance encoders, is required to position the stage in a desired location. One of the problems with using encoders, rather than, for example, interferometers, is the lack of resolution. Interferometers can resolve movement as small as a wave-length of the laser source. Encoders require a scale or scales with optically resolvable lines or hatching. It is difficult to make very fine lines over a large area to achieve a resolving power even remotely approaching that of an interferometer. Interferometers are expensive and limit the speed with which the traversing system can operate. Resolving power is always traded against cost in designing manufactured systems. It is a goal in this industry to achieve high resolving power of the positioning information with low cost and high speed. Any increase in the cost-effectiveness of high-resolution systems is highly desirable.
To use two linear scales, one for each axis of movement, a scale may be placed on the base and a corresponding optical pickup on the gantry. The other scale may be placed on the gantry and another optical pickup on the stage. This system may require greater stiffness and precise alignment of the gantry mechanism because of the distance between the encoder scale and the stage, assuming the encoder scale is mounted on the edge of the base. Another method of movement encoding is to use X- and Y-optical pickups moving over a grid of lines, instead of a linear series of lines.
Referring to FIG. 1 a traversing machine 1, according to the prior art, uses a single grid-scale 2. Traversing machine 1 includes a gantry 7 with track 7. Track 7 includes rails 8. Movable stage 6 has linear bearings 5 that couple movable stage 6 with rails 8 to permit movable stage 6 to ride along rails 8. Gantry 18 travels on rails 17 in a direction perpendicular to rails 8.
A cross-hatch pattern of lines 4, a respective set being parallel to the X-axis and a respective set being parallel the Y-axis, define a rectangular array of square patches 3, lines 4 are formed in a surface over which the stage flies. An X-direction optical pickup 5a detects light reflected from grid lines so as to be sensitive only to movement in the X-direction. A Y-direction optical pickup 5b detects light reflected from grid lines so as to be sensitive only to movement in the Y-direction.
Stage 6 moves in X and Y directions aligned with respective perpendicular edges of base 22. The means by which X-Y stage 6 moves relative to base 22 could be any of a number of different known mechanisms (not shown), such as an air bearing, a gantry mechanism mounted on linear bearings, etc. U.S. Pat. No. 5,334,892 describes a traversing system in which stage rests on air bearings that float on a magnet platen. Motor armatures on the stage generate changing magnetic fields that interact with fields generated by magnets in the magnet platen to create a motive force. In this case position and orientation information are supplied by interferometers that bounce laser beams off mirrors attached to the platen. Interferometer-based systems require expensive vibration isolation in order to work properly. In addition, interferometers are more expensive than optical pickups used in other types of traversing systems.
Problems with applying the encoder technology of traversing systems are numerous and varied. There is a pressing need to discriminate very fine degrees of movement. However, as is readily apparent, manufacturing a grid scale with very fine pitch is costly. Generally, such scales are made of a reflective material with machined or printed portions formed on their surfaces. Moreover, creating traversing machinery that provides optical pickups with access to a stationary grid scale without interference is another obstacle to design. Yet another problem is economically printing oil a large area to produce an accurate and consistent grid scale. Still another problem is the vulnerability of highly precise scales to incidental damage or occasional breakdown of the traversing system. Still another problem is cost-effective production of an entire motor platen, with a precise flat surface, especially one with a built-in optical scale.